In a nutshell, the Mayans, who had advanced systems for language, astronomy, and time-keeping, had a measurement called a 'baktun' which was 144,000 days, and the 12th baktun ends on December 21, 2012, and the 13th baktun begins. People have made a big deal of it but it's nothing different really than when a century ends and a new one begins.
